Van 't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for NanoMaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
J Chem Phys. 2011 Jan 28;134(4):044904. doi: 10.1063/1.3520389.
We investigate the effect of a magnetic field on the shape and director field of nematic droplets in dispersions of sterically stabilized and charge-stabilized colloidal gibbsite platelets with a negative diamagnetic anisotropy. Depending on the magnetic field strength and tactoid size, we observe with polarized light microscopy several interesting structures, with different shapes and director fields both with and without defects. In particular, our findings provide the first experimental evidence for the existence of the split-core defect structure predicted ten years ago by Mkaddem and Gartland [Phys. Rev. E 62, 6694 (2000)]. The split-core structure is a metastable director-field configuration that can be stabilized by a sufficiently strong externally applied magnetic field but only if the diamagnetic anisotropy of the particles is negative. To account for our observations, we present a calculation of the stability regions of different shapes and director-field structures as a function of tactoid size, anchoring conditions, surface tension, elastic constants, and magnetic field strength. By fitting the experimental data to the theoretically predicted structures, we are able to extract values for the splay elastic constant, interfacial tension, and anchoring strength. Remarkably, we find significant differences between the two systems studied: for sterically stabilized gibbsite in bromotoluene the anchoring strength is one order of magnitude larger than that of aqueous gibbsite, with the latter exhibiting weak and the former strong anchoring of the director field to the interface. The splay elastic constants that we obtain are in agreement with earlier experiments, simulations, and theory, while the interfacial tension and anchoring strength are considerably larger than what was found in earlier experiments.
我们研究了磁场对具有负反磁各向异性的空间稳定和电荷稳定胶体水铝石板分散体中向列滴的形状和指向矢场的影响。根据磁场强度和向列畴的大小，我们用偏光显微镜观察到了几种具有不同形状和指向矢场的有趣结构，包括有缺陷和无缺陷的结构。特别是，我们的发现为 Mkaddem 和 Gartland 十年前预测的分裂核缺陷结构的存在提供了第一个实验证据[Phys. Rev. E 62, 6694 (2000)]。分裂核结构是一种亚稳的指向矢场构型，可以通过足够强的外部磁场稳定，但前提是颗粒的反磁各向异性为负。为了解释我们的观察结果，我们提出了一种计算不同形状和指向矢场结构的稳定区域的方法，作为向列畴大小、锚定条件、表面张力、弹性常数和磁场强度的函数。通过将实验数据拟合到理论预测的结构中，我们能够提取出伸展弹性常数、界面张力和锚定强度的值。值得注意的是，我们发现所研究的两个系统之间存在显著差异：对于溴甲苯中的空间稳定水铝石，锚定强度比水相水铝石大一个数量级，后者表现出较弱的指向矢场对界面的锚定，而前者表现出较强的锚定。我们得到的伸展弹性常数与早期的实验、模拟和理论一致，而界面张力和锚定强度比早期实验中发现的要大得多。
